The present invention generally relates to synchronous generators, and more specifically, to an apparatus and method for providing generator protection for a synchronous electrical generator in a power system.
Synchronous electrical generators (“synchronous generators”) are used in many applications requiring alternating current (AC) power generation. For example, electric utility systems or power systems include a variety of power system elements such as synchronous generators, power transformers, power transmission lines, distribution lines, buses, capacitors, etc. to generate, transmit and distribute electrical energy to loads. A synchronous generator operates to, for example, convert mechanical rotation via a prime mover (e.g., shaft rotation provided by a coal powered steam turbine) into AC current via electromagnetic principles. After suitable conditioning, the alternating electrical current is transmitted and distributed as three-phase electric power to a variety of loads.
As is known, synchronous generator design is based on Faraday's law of electromagnetic induction and includes a rotational portion for inducing an electromotive force (EMF) in a stationary portion. The rotational portion is driven by the prime mover. More specifically, the rotational portion, or rotor, includes a field winding wrapped around a rotor body, and the stationary portion includes a stator having an armature winding. The rotor body, typically made of steel, may have a salient pole structure (i.e., poles protruding from a shaft) or a cylindrical structure.
In operation, EMFs are induced in the armature windings of the stator upon application of DC current to the field winding of the rotor. That is, direct current is made to flow in the field winding. This results in a magnetic field, and when the rotor is made to rotate at a constant speed, the magnetic field rotates with it. Accordingly, as the moving magnetic field passes through the stator winding(s), an EMF is induced therein. If the stationary armature includes, for example, three stationary armature windings, they experience a periodically varying magnetic field, and three EMFs are induced therein. These three EMFs conform a three-phase system of voltages. Thus, for 60-Hz AC systems, in a two-pole machine, the rotor has to rotate at 3600 revolutions per minute with three armature windings displaced equally in space on the stator body to generate three-phase electric power.
As the generator electric load increases, the generator demands more mechanical power from its prime mover and more current flows through the stator winding, and therefore more electric active power is delivered from the synchronous generator to the power system. By increasing the current to the rotor winding, the synchronous generator produces more reactive power, also called reactive volt-amperes (VARs), which, in effect, can raise the power system voltage. Conversely, by decreasing the current in the rotor winding, VARs are absorbed by the generator, effectively lowering the power system voltage. As is known, we express in Watts or Megawatts the active power delivered to or consumed by a load, while VAR or MVAR is the imaginary counterpart of the Watt or Megawatt and represents the reactive power consumed or generated by a reactive load (i.e., a load having a phase difference between the applied voltage and the current).
Generator capability curves (“capability curves”) are typically provided by a generator manufacturer to define the operating or thermal limits of a particular synchronous generator at different cooling pressures. Each capability curve represents the synchronous generator capability limit for a pressurized coolant (e.g., hydrogen) circulating to cool the stator and rotor windings. More cooling enables more armature current to flow during synchronous generator operation, while less cooling enables less current to flow. Additionally, over excitation limiter (OEL) curves and minimum excitation limiter (MEL) curves are typically included with the manufacturer-provided capability curves. Steady state stability limit (SSSL) curves may further be determined with generator impedance data and power system parameters.
Because there are limits to the amount of current that can flow through the stator and rotor winding, the operating limits reflected in the capability curves are imposed on the amount of Watts and VARs that the synchronous generator can deliver to the power system. There is also a minimum value of current that must flow in the rotor field to maintain generator stability, and this imposes a limit on the amount of VARs that the synchronous generator can absorb for each delivered active power value. Thus, the operating limits graphically illustrated by the capability curve(s) include an active power component “P” expressed in Megawatts (MW) and a reactive power component “Q” expressed in Mega VARs (MVARs). As long as the P, Q operating point of the synchronous generator (i.e., as long as the amount of Watts and VARs flowing out of or into the generator) is within its safe operating limits, or inside its capability curve, the synchronous generator will operate within safe limits.
Although the operating limits defined by capability curves are utilized by power generating station operators to ensure safe synchronous generator operation, it has been suggested to utilize these curves to influence excitation control of a synchronous generator in real time. For example, U.S. Pat. No. 5,264,778, entitled “Apparatus Protecting a Synchronous Machine from Under Excitation,” issued on Nov. 23, 1993, describes a microprocessor based voltage regulator system that provides a minimum limit on excitation that is defined using one or more straight line segments approximating the associated machine capability curves. Such a minimum limit on excitation prevents the excitation of the synchronous generator from falling below a predetermined P-Q characteristic. The microprocessor based voltage regulator system of the U.S. Pat. No. 5,264,778 is included in a control system of the synchronous generator.
It has also been suggested that synchronous generator operation may be improved via use of a visual display that reflects synchronous generator operation with respect to its capability curves. U.S. Pat. No. 5,581,470, entitled “Apparatus for Visually Graphically Displaying the Generator Point of a Generator in Reference to its Capability Curve Including Digital Readouts of Watts, VARs and Hydrogen Pressure,” issued on Dec. 3, 1996, describes a computer-based meter that provides a real time graphical display which visually indicates an operating point in relation to a capability curve(s) of a synchronous generator during operation. The operating point(s) and capability curves are defined and displayed based on measurement signals from Watt, VAR and hydrogen pressure transducers.
Synchronous generator outages or failures due to power system faults, abnormal operating conditions, and the like, can be some of the costliest in the power system. Accordingly, protective devices are operatively coupled to the synchronous generators and their outputs in order to measure currents and voltages indicative of synchronous generator operation. Such protective devices are referred to hereinafter as protective relays, and typically include a variety of protective functions or elements.